There is considerable interest in developing sensors that act as analogs of the mammalian olfactory system (Lundstrom et al. (1991) Nature 352:47–50; Shurmer and Gardner (1992) Sens. Act. B 8:1–11; Shurmer and Gardner (1993) Sens. Actuators B 15:32). Prior attempts to produce broadly responsive sensor arrays have exploited heated metal oxide thin film resistors (Gardner et al. (1991) Sens. Act. B4:117–121; Gardner et al. (1991) Sens. Act. B 6:71–75), polymer sorption layers on the surfaces of acoustic wave (SAW) resonators (Grate and Abraham (1991) Sens. Act. B 3:85–111; Grate et al. (1993) Anal. Chem. 65:1868–1881), arrays of electrochemical detectors (Stetter et al. (1986) Anal. Chem. 58:860–866; Stetter et al. (1990) Sens. Act. B 1:43–47; Stetter et al. (1993) Anal. Chem. Acta 284:1–11), conductive polymers or composites that consist of regions of conductors and regions of insulating organic materials (Pearce et al. (1993) Analyst 118:371–377; Shurmer et al. (1991) Sens. Act. B 4:29–33; Doleman et al. (1998) Anal. Chem. 70:2560–2654; Lonergan et al. Chem. Mater. 1996, 8:2298). Arrays of metal oxide thin film resistors, typically based on tin oxide (SnO2) films that have been coated with various catalysts, yield distinct, diagnostic responses for several vapors (Corcoran et al. (1993) Sens. Act. B 15:32–37). Surface acoustic wave resonators are extremely sensitive to both mass and acoustic impedance changes of the coatings in array elements, but the signal transduction mechanism involves somewhat complicated electronics, requiring frequency measurement to 1 Hz while sustaining a 100 MHZ Rayleigh wave in the crystal. Attempts have also been made to construct arrays of sensors with conducting organic polymer elements that have been grown electrochemically through use of nominally identical polymer films and coatings. Moreover, Pearce et al., (1993) Analyst 118:371–377, and Gardner et al., (1994) Sensors and Actuators B 18–19:240–243 describe, polypyrrole based sensor arrays for monitoring beer flavor. U.S. Pat. No. 4,907,441, describes general sensor arrays with particular electrical circuitry. U.S. Pat. No. 4,674,320 describes a single chemoresistive sensor having a semi-conductive material selected from the group consisting of phthalocyanine, halogenated phthalocyanine and sulfonated phthalocyanine, which was used to detect a gas contaminant. Other gas sensors have been described by Dogan et al., Synth. Met. 60, 27–30 (1993) and Kukla, et al. Films. Sens. Act. B., Chemical 37, 135–140 (1996).
Typically, the detectors in such an array are placed in nominally spatially equivalent positions relative to the analyte flow path. In such a configuration, any spatiotemporal differences between detectors are minimized, and the array response pattern is determined by the differing physicochemical responses of the various detectors towards the analyte of interest. The variations in analyte sorption amongst various detectors thus determines the resolving power of the detector array and determines the other performance parameters of such systems.
Additionally, the form factor of the individual detectors in such arrays is typically constrained by factors related to the mode of signal transduction. For example, most film-coated quartz-crystal microbalance (QCM) devices must have specified dimensions so that a resonant bulk acoustic wave can be maintained in the quartz crystal transducer element. Similarly, the geometry of SAW devices is constrained by the need to sustain a Rayleigh wave of the appropriate resonant frequency at the surface of the transducer crystal. Each detector in a QCM or SAW array typically has an identical area and form factor; consequently, the array response is based solely on the different polymer/analyte sorption properties of the differing detector films.
In practice, most chemical sensors suffer from problems associated with mass transport of the analyte to be detected to the sensor regardless of the type of detector or sensor.